Liverpool have been strongly linked with a move for the 25-year-old defensive midfielder after he scored against Brazil at this summer's World Cup, with Arsenal and Tottenham Hotspur also credited with interest.
Speculation over Sano's future was put to Heidel in a recent interview with BILD, and while he declined to be drawn on any of the specific clubs credited with an interest, he was candid about how Mainz typically handle transfer business.
"You know we like to prepare transfers in such a way that the public only finds out once we announce them," he said. "And usually, we manage to do just that! I won't comment on any rumours or discuss any of the clubs mentioned."
Heidel stressed that Mainz are under no financial pressure to cash in on one of their prized assets this window.
"We don't have to sell Kaishu; we can afford to keep him. If he stays, there will be a lot of happy people in the squad, the club, and the surrounding community," he said.
"But it's the same as always at Mainz 05: if an offer comes along that makes us gasp for breath, we have to consider it!"
Speaking separately to BILD, comments later relayed by Sport Witness, Heidel acknowledged the growing scale of interest in his midfielder and did not rule out a switch to the Premier League specifically.
"There's no doubt there are interested parties," he said. "It's well known that England is the league where outstanding players usually move. Our big advantage is that the player still has a long-term contract, so we're under no pressure at all and won't be rushing anything.
"If a club comes along with an offer, we'll consider it. If, in our view, it's not high enough for the player's market value, then it's also possible that Kaishu will stay with us longer. As of today, everything is still open. But we know, of course, that we have a player who is generating a lot of interest."
Sano joined Mainz in 2024 and has developed into one of the Bundesliga's most reliable defensive midfielders, a rise capped by his performances for Japan at the World Cup this summer, where his goal against Brazil significantly raised his profile among Europe's leading clubs.